62.93 62.95 62.97 62.94 62.98 62.98 62.95 Mean, 62.957, .005 Second. A warm aqueous solution of selenious acid was mixed with HCl, and reduced by a current of SO2. The reduced Se was collected upon a glass filter, dried, and weighed. Percentages of Se in SeO2 : 71.199 71.185 71.193 71.187 71.191 Mean, 71.191, ± .0016 This series, combined with that of Sacc, 71.088, ± .032, gives a general mean of 71.1907, .0016. There are now five series of figures from which to deduce the atomic weight of selenium: (1.) Per cent. of Se in SeO2, 71.1907, ± .0016 (2.) BaSeO, BaSO:: 100: 88.437, ± .013 (5.) Per cent. of Ag in Ag2SeO3, 62.957, ±.005 From these we get the following values for selenium: TELLURIUM. Particular interest attaches to the atomic weight of tellurium, on account of the speculations of Mendelejeff. According to the "periodic law" of that chemist, tellurium should lie between antimony and iodine, having an atomic weight greater than 120, and less than 127. Theoretically, Mendelejeff assigns it a value of Te = 125; but all the published determinations lead to a mean number higher than would be admissible under the aforesaid "periodic law." Whether theory or experiment is at fault remains to be discovered. The first, and for many years the only, determinations of the constant in question, were made by Berzelius.* By means of nitric acid he oxidized tellurium to the dioxide, and from the increase in weight deduced a value for the metal. He published only his final results; from which, if 0100, Te 802.121. The three separate experiments give Te 801.74, 801.786, and 802.838; whence we can calculate the following percentages of metal in the dioxide: = 80.057 80.034 Mean, 80.042, .005 The next determinations were made by von Hauer,† who resorted to the analysis of the well crystallized double salt TeBr,.2KBr. In this compound the bromine was estimated as silver bromide, the values assumed for Ag and Br being respectively 108.1 and 80. Recalculating, with our newer atomic weights for the above named elements, we get from v. Hauer's analyses, for 100 parts of the salt, the quantities of AgBr which are put in the third column: * Poggend. Annal., 28, 395. 1833. 2.000 grm. K,TeBr, gave 69.946 per cent. Br. 164.460 From Berzelius' series we may calculate Te = 128.045, and from v. Hauer's Te 127.419. Dumas,* by a method for which he gives absolutely no particulars, found Te = = 129. In 1879, with direct reference to Mendelejeff's speculations, the subject of the atomic weight of tellurium was taken up by Wills. The methods of both Berzelius and von Hauer were employed, with various rigid precautions in the way of testing balance and weights, and to ensure purity of material. In the first series of experiments tellurium was oxidized by nitric acid to form TeO,. The results gave figures ranging from Te 126.31 to 129.34: = 2.21613 grm. Te gave 2.77612 grm. TeO2. 79.828 per cent. Te. In the second series tellurium was oxidized by aqua regia to TeO2; with results varying from Te 127.77 to 128.00: = 2.85011 grm. Te gave 3.56158 grm. TeO. 80.024 per cent. Te. Combining these series with that due to Berzelius, we have the following general mean: * Ann. d. Chim. et d. Phys., (3,) 55, 129. 1859. By von Hauer's process, the analysis of TeBr..2KBr, Will's figures give results ranging from Te = 126.07 to 127.61. Reduced to a common standard, 100 parts of the salt yield the quantities of AgBr given in the third column: 1.70673 grm. K2TeBr ̧ gave 2.80499 grm. AgBr. 164.349 Combined with von Hauer's mean, 164.408, ± .045, this gives a general mean of 164.468, .033. Hence Te= 127.170, .173. A careful consideration of the foregoing figures, and of the experimental methods by which they were obtained, will show that they are not absolutely conclusive with regard to the place of tellurium under the periodic law. The atomic weight of iodine, calculated in a previous chapter, is 126.557. Wills' values for Te, rejecting his first series as relatively unimportant, range from 126.07 to 128.00; that is, some of them fall below the atomic weight of iodine, although none descend quite to the 125 assumed by Mendelejeff. In considering the experimental methods, reference may properly be made to the controversy regarding the atomic weight of antimony. It will be seen that Dexter, estimating the latter constant by the conversion of the metal into Sb,O,, obtained a value approximately of Sb = 122. Dumas, working with SbCl,, obtained a similar value. Schneider and Cooke, on the other hand, have established an atomic weight for antimony near 120, and Cooke in particular has traced out the constant errors which lurked unsuspected in the work of Dumas and Dexter. Now in some physical respects tellurium and antimony are quite similar. As constant errors vitiated the recently accepted values for Sb, so they may also effect our estimates for Te. The oxidation of Te by nitric acid resembles in minor particulars that of Sb. The analysis of K, TeBr,, gives a low value for Te, and yet the material may have contained traces of oxybromides, the presence of which would render even that lower value too high. A careful revision of the atomic weight of tellurium is still necessary. VANADIUM. 2 3 Roscoe's determination of the atomic weight of vanadium is the only one having any present value. The results obtained by Berzelius* and by Czudnowicz † are unquestionably too high; the error being probably due to the presence of phosphoric acid in the vanadic acid employed. This particular impurity, as Roscoe has shown, prevents the complete reduction of V2O, to V2O, by means of hydrogen. All vanadium ores contain small quantities of phosphorus, which can only be detected with ammonium molybdate; a reaction unknown in Berzelius' time. Furthermore, the complete purification of vanadic acid from all traces of phosphoric acid is a matter of great difficulty, and probably never was accomplished until Roscoe undertook his researches. In his determination of the atomic weight, Roscoe Poggend. Annal., 22, 14. 1831. Journ. Chem. Soc., 6, pp. 330 and 344. 1868. |